Casting steel strip

Abstract

In twin roll casting of steel strip, molten steel is introduced into the nip 16B between parallel casting rolls to create casting pool supported on casting surfaces of the rolls and the rolls are rotated to deliver solidified strip downwardly from the nip. Casting surfaces are textured by a random pattern of discrete projections, which may have an average surface distribution of between 5 and 200 peaks per mm 2 and an average height of at least 10 microns. In order to suppress chatter defects, the molten steel also has manganese content of at least 0.55% by weight and a silicon content in the range of 0.1 to 0.35% by weight. The strip is thus capable of moving away from the casting pool at a speed of more than 60 meters per minute without substantial high speed chattering defects.

The invention claimed is: 
     
       1. A method of continuously casting steel strip comprising:
 (a) providing a chilled casting surface with a texture formed by a random height distribution of discrete projections; 
 (b) contacting chilled casting surface having such formed texture with a casting pool of molten steel having a manganese content of at least 0.55% by weight and a silicon content in the range of 0.1 to 0.35% by weight to cause solidification of steel from the casting pool onto the casting surface as a solidified shell; and 
 (c) separating the solid shell from the casting surface in a solidified strip. 
 
     
     
       2. A method as claimed in  claim 1 , wherein the steel has a carbon content of less than 0.07% by weight. 
     
     
       3. A method as claimed in  claim 1 , wherein at least some of said discrete projections have an average surface distribution of between 5 and 200 peaks per mm 2 . 
     
     
       4. A method as claimed in  claim 1 , wherein said discrete projections have an average height of at least 10 microns. 
     
     
       5. A method as claimed in  claim 4 , wherein the average height of the discrete projections is at least 20 microns. 
     
     
       6. A method as claimed in  claim 1 , comprising the additional step of the strip moving away from the casting pool at a speed of at least 60 meters per minute. 
     
     
       7. A method as claimed in  claim 6 , wherein the strip is moved away from the casting pool at a speed in the range of 75 meters per minute. 
     
     
       8. A method as claimed in  claim 1 , wherein the manganese content of the steel is in the range of 0.55 to 0.9% by weight. 
     
     
       9. A method of continuously casting steel strip comprising:
 (a) forming a pair of casting rolls having casting surface being textured by a random height distribution of discrete projections, 
 (b) introducing molten steel having a manganese content of at least 0.55% by weight and a silicon content in the range of 0.1 to 0.35% by weight into a nip between said pair of casting rolls having such formed texture that are chilled to form a casting pool of the molten steel supported on the casting surfaces of the rolls immediately above the nip; and 
 (c) rotating the rolls to cause solidified steel shells forming on the casting surfaces in contact with the casting pool to be brought together into a solidified steel strip delivered downwardly from the nip. 
 
     
     
       10. A method as claimed in  claim 9 , wherein said discrete projections have an average surface distribution of between 5 and 200 peaks per mm 2  and an average height of at least 10 microns. 
     
     
       11. A method of continuously casting steel strip comprising:
 (a) forming a pair of casting rolls having casting surface being textured by a random height distribution of discrete projections, wherein each casting surface is defined by a grit blasted substrate covered by a protective coating such that the casting surface shows the random distribution texture of discrete projections, 
 (b) introducing molten steel having a manganese content of at least 0.55% by weight and a silicon content in the range of 0.1 to 0.35% by weight into a nip between said pair of casting rolls having such formed texture that are chilled to form a casting pool of the molten steel supported on the casting surfaces of the rolls immediately above the nip; and 
 (c) rotating the rolls to cause solidified steel shells forming on the casting surfaces in contact with the casting pool to be brought together into a solidified steel strip delivered downwardly from the nip. 
 
     
     
       12. A method as claimed in  claim 11 , wherein the protective coating is an electroplated metal coating. 
     
     
       13. A method as claimed in  claim 12 , wherein the substrate is copper and the plated coating is of chromium. 
     
     
       14. A method as claimed in  claim 9 , wherein each casting surface is grit blasted. 
     
     
       15. A method as claimed in  claim 14 , wherein the grit blasted surface is formed of nickel. 
     
     
       16. A method as claimed in  claim 9 , wherein each casting surface is defined by a coating deposited onto a substrate to produce the random distribution texture of that surface. 
     
     
       17. A method as claimed in  claim 16 , wherein the coating is formed by chemical deposition. 
     
     
       18. A method as claimed in  claim 16 , wherein the coating is formed by electrodeposition. 
     
     
       19. A method as claimed in  claim 16 , wherein the coating is formed of a material which has a low affinity for the oxidation products in the molten steel such that the molten steel has greater affinity for the coating material and wets the coating in preference to said oxidation products. 
     
     
       20. A method as claimed in  claim 16 , wherein the coating is formed of an alloy of nickel, chromium and molybdenum. 
     
     
       21. A method as claimed in  claim 16 , wherein the coating is formed of an alloy of nickel, molybdenum and cobalt.